(1)Notice that if we make the sun 100 inches in diameter, the earth will be merely one inch
in diameter and will be located nearly 900 feet (three football fields) away from the sun!

(2)Notice, too, that Proxima Centauri (the closest star, other than the sun)would be 46,000
miles away!

(3)Why are the solar system pictures shown in books so misleading? Because there is no
way to picture the solar system on a single page of a book, and still have the planets be
visible!

(4)To fit on the same 8-inch wide page, the sun would have to be 0.07 inches wide and the
earth would end up .0006 inches wide. Mars, Jupiter, Saturn, and Uranus wouldn’t even
make it onto the page. (Jupiter, for example, would be off the page by 3 feet.)

d.The Solar System is mostly nothing. It took 33 years for the Voyager spacecraft, launched in
1977, to reach the edge of the solar system. If it continued to travel at its current speed, it would
take 93,000 years for it to reach Proxima Centauri.

b.To understand the point of this article, go to the black rectangles showing stars, left click on the
little white box and WHILE STILL HOLDING DOWN YOUR CLICK, drag the white box to the
right; it will show you what a constellation would look like from different points of view.

3.The Milky Way (= our galaxy):

a.A galaxy is a collection of stars.

i.The Milky way is estimated to have 200 to 400 billion stars. It is an estimation because we
can’t see them all.

(1)With the naked eye, we can see only perhaps 8,000 stars, and fewer than half of these
will be visible at any one time, since the others will be blocked from our view by the
earth.

(a)In Dayton, thanks to the light pollution, you won’t be able to see many stars at all.

(2)Even with a powerful telescope, we can’t see all the stars in our galaxy. Some we can’t
see because our view is blocked by dust and gas. Others we can’t see because other stars
block them from our view. Yet others are sufficiently dim and distant that we can’t see
them.

(a)Proxima centuri is the closest star to us (other than the sun), but it can’t be seen with
the naked eye since it is rather dim. The nearest star that can be seen with the naked
eye is Alpha centuri, but in seeing it you are actually cheating a bit, since what you
see is in fact two stars (Proxima centuri A and B) instead of one. The stars in
question are a binary pair, and revolve around each other the way Jupiter revolves
around the sun. Not only that, but Proxima centuri is thought to revolve around these
two stars, making it a triple star system.

(b)Although our sun isn’t part of a binary system, it is thought that most stars are.
Polaris (the north star), for example, is not a unitary star, but is instead part of a five
star system!

(3)If you look at the sun with a telescope (or even with the naked eye), it is a disk with
angular diameter. There are only a few other stars, though, with measurable angular
diameters. The rest appear as points of light, even when seen through the most powerful
telescopes. This is because they are so very far away.

(4)It is thought that most of the stars in our galaxy—indeed, most of the stars in the
universe—have planets orbiting them. The Kepler space telescope is routinely
discovering them.

(a)It usually can’t see them individually, the way we might see Jupiter in a backyard
telescope. But their effect on the star they are orbiting can be detected. The star in
question might wobble because of the planet’s pull, for example. Or the star might
grow slightly dimmer when the planet passes between us and it.

i.Any star you can see with your naked eye will be within our galaxy—indeed, it will be fairly
close by in our galaxy. There will be many stars within our galaxy that you can’t see
individually; instead, the region they are in will look “milky.” Another galaxy, Andromeda,
can be seen with the naked eye if you are in a very dark area and know just where to look; it
will look like a lighter patch.

ii.There are two exceptions to the rule that you can see, with your naked eye, only stars within
the Milky Way:

(1)One exception is if there is a supernova explosion in another galaxy. This can result in a
star being visible from the earth—more precisely, result in its remnants being visible.
Thus, in 1987, there was a naked-eye supernova in the Large Magellanic Cloud, which is
a dwarf galaxy that, although separate from the Milky Way, is gravitationally bound to it.

(a)It is interesting that three hours before the light from the 1987 supernova became
visible, neutrino detectors recorded an influx of neutrinos. This is because the
process of stellar collapse that produces a supernova releases neutrinos before the
final explosion.

(b)This was the first naked-eye supernova since 1604, when Johannes Kepler saw one.
That supernova, unlike the 1987 supernova, was within our own galaxy.

(c)Another famous naked-eye supernova took place in 1054 (apparently on the 4th of
July). Chinese and Japanese astronomers recorded it. The remnant of this supernova
can still be seen. It is known as the Crab Nebula:
http://www.nasa.gov/images/content/430450main_image_1604_946-710.jpg
Astronomer’s measured how fast the gas in this nebula is expanding outward, and
when they used this data to determine when it “exploded,” they got results
compatible with the 1054 date. The Crab Nebula is also in the same part of the sky
as the 1054 supernova was recorded as having taken place.

(2)The second exception is gamma-ray bursts. Seven-and-a-half billion years ago, a
collapsing star emitted such a burst, and if you had been looking at the right point in the
sky on March 19, 2008, for the right 30-second period, you would have seen a “star”
from a very distant galaxy flare into existence and then disappear. See
http://news.nationalgeographic.com/news/2008/03/080321-brightest-object.html

iii.Stars in other galaxies can be seen with powerful telescopes, but it wasn’t until the early
1920s that astronomer’s had telescopes powerful enough to perform this feat. Indeed, it was
only in 1925 that astronomers became confident of the existence of galaxies other than our
own!

i.Notice that in the same way as we are not at the center of the solar system, we are not at the
center of our galaxy.

(1)Question: if we have never been outside our galaxy, how can we know what it looks lik e
from outside? (A student could draw a map of the classroom without leaving his seat.)

ii.It is actually a good thing that the sun is not at the middle of our galaxy, since there is
evidence that a supermassive black hole, 4 million times the mass of the sun, resides there
and would make our existence rather miserable.

iii.The insignificance of the sun: If we were 56 light years away from the sun, it would be so
dim that we could no longer see it with our naked eye (but could still find it with a
telescope). The Milky Way is itself 100,000 light years across. Suppose, then, that we made
a poster showing the Milky Way and that on this poster, the Milky Way was 14 inches
across. Suppose that we indicated the position of the sun within the Milky Way by a dot the
size of a (1/64th of an inch wide) period. The circumference of this period would also
represent the distance from which the sun would be observable with the naked eye!

i.Realize that in the same way as the Solar System is mostly nothing, and the Milky Way is
mostly nothing, the space between the Milky Way and Andromeda is remarkably empty.
Indeed, if we could travel at the speed of light, we would spend 2.5 million years traveling
through empty space before we reached Andromeda. Conclusion: the universe is truly huge,
and it is filled with mostly nothing.

i.When galaxies collide, though, the stars within them are unlikely to collide as well; there is
simply too much space between stars for this to happen.

g.The universe is full of galaxies: The Hubble Ultra Deep-Field view.

i.During 400 orbits between September 24, 2003, and January 16, 2004, the Hubble telescope
was pointed at the same tiny patch of sky. It made two exposures per orbit, and in the 800
exposures that resulted, the total exposure time was 11.3 days, or nearly a million seconds.
This allowed lots of light to accumulate, the way keeping an old-fashioned camera lens open
for a very long time would.

ii.This long exposure time allowed the telescope to see extremely dim objects. Indeed, some
of the objects were so dim that the telescope collected only one photon of light per minute
from them. By way of contrast, it collects photons from nearby galaxies at the rate of
millions per minute.

iii.The area of sky in the photograph was chosen because it contained no stars that are visible to
the naked eye or even to a moderately powerful telescope. It is an area of the sky as big as
1/50 of the full moon. And just so you are clear about how big this area is, hold your hand at
arm’s length with the back of your hand facing you. Look at the nail of your little finger.
That nail is big enough to cover the full moon! The area shown in the Hubble Ultra Deep-Field view is 1/50th that large!

h.One important thing to realize about the Hubble Ultra-Deep and Extreme-Deep Views is that no
matter where they pointed the telescope, they would see the same thing. (Although point it in
some directions, and dust or nearby stars would impede the viewing.) The scientific term for this
is that the universe, seen from our point of view, is isotropic: it looks the same in every direction.
(At least it does when you look at very distant objects; look at closer objects, and it is not
isotropic. There is, for example, more of the Milky Way on one side of us than on the other.)

i.This is a significant fact. If I had students draw a map of the classroom and locate
themselves on that map, those who were near the walls would see more people on one side of
them than the other: their view wouldn’t be isotropic. Only those in the middle would see
about the same number of people no matter which direction they looked.

i.Someone might conclude that from the fact that the universe looks isotropic, we must be in the
middle of it. (This is some consolation after finding out that we are in the middle of neither our
solar system nor our galaxy.)

i.This conclusion can be challenged, though. Scientists are convinced that besides being
isotropic, our universe is homogeneous. (According to the cosmological principle, the
universe is both isotropic and homogeneous.) Wherever you are, you will see the same
isotropic distribution of galaxies. How can this be?

(1)One way is if the universe is infinite.

(2)Another way is if space is curved. To understand this, imagine the predicament of a
sailor on a planet that was completely submerged by water. Things would look the same
in every direction—it would be isotropic—and no matter where he traveled, things
would still be isotropic—meaning that it would be homogenous as well.

i.Notice the “voids” in this image. The universe if full of areas full of nothing. The space
between the sun and planets is full of nothing, the space between stars is full of nothing, and
the space between galaxies is full of nothing. Some of these spaces between galaxies are
quite large and are known as voids. The largest known void is the Eridanus Supervoid which
is a billion light years across.

7.It is possible that there are universes other than our own—that our universe is but one of many
multiverses—in which case the last location in your address should be “This Universe” rather than
simply “Universe.”